1. Field of the Invention
The present invention relates to a method and apparatus for detecting a fault in an exhaust gas oxygen sensor.
2. Description of the Related Art
Controlling emissions from automobile engines has for some time been an important focus in the design of automobiles. Emission control is receiving increased attention with upcoming On-Board Diagnostics II (OBD II) emission regulations.
In vehicle emission control systems, important components are exhaust gas oxygen (EGO) sensors. A vehicle may have one or more EGO sensors. These sensors measure the amount of oxygen in a vehicle's exhaust. The outputs of these sensors are typically fed back to an electronic engine controller. The electronic engine controller in turn controls a number of engine parameters to attempt to keep the engine running at a desired intake air-fuel ratio. Successful maintenance of the desired intake air-fuel ratio helps reduce undesirable emissions from the engine.
In normal operation, a typical EGO sensor generates an output signal which transitions between two voltage levels. One voltage level is generated when there is excess oxygen in the vehicle exhaust, an indicator that the engine is running at a lean air-fuel ratio. The other voltage level is generated by the EGO sensor when there is a lack of oxygen in the vehicle exhaust, an indicator that the engine is running at a rich air-fuel ratio. In normal engine operation, the output of an EGO sensor will frequently switch between the two voltage levels, as the electronic engine controller continually strives to maintain the desired intake air-fuel ratio.
Typically, an EGO sensor has a reference electrode located in a port which is open to the atmosphere. This electrode, because it is exposed to the atmosphere, is exposed to a relatively constant and known amount of oxygen. This relatively constant amount of oxygen serves as a reference against which the vehicle exhaust gas is compared. An EGO sensor can thus generate an output signal indicative of the oxygen content of the vehicle exhaust gas.
Because automotive emission control is such an important endeavor and because EGO sensors play such an important part in automotive emission control, diagnosing faults in an EGO sensor output signal is important. Faults in an EGO sensor output signal can have a number of causes and can be manifested by a number of effects on the EGO sensor output signal. For example, the electrical wiring which carries an EGO sensor output signal to the electronic engine controller can become short-circuited or open-circuited, resulting in an overvoltage or undervoltage EGO sensor signal. In addition, air leaks in the vehicle exhaust system can cause an EGO sensor to generate a faulty signal. Also, a phenomenon known as characteristic shift downward (CSD) can occur. A common cause of CSD is contaminants which enter the atmospheric oxygen port of an EGO sensor and reduce the exposure of the reference electrode to atmospheric oxygen. When such contamination occurs, the voltage of the EGO sensor output signal shirts downward. For example, instead of the EGO sensor operating between approximately zero and one volt, the EGO sensor output signal may shift downward and instead operate between approximately -1 and zero volts.
When a fault in an EGO sensor output signal occurs, it is important to detect that fault so the vehicle's owner can be notified by the electronic engine controller to bring the vehicle into a dealership for repair. However, it is also advantageous to be able to distinguish between the various causes of EGO sensor output signal faults. Different causes may require different actions by a repair technician (for example, fixing a shorted wire, fixing an air leak in the vehicle's exhaust system, or replacing the EGO sensor).
As mentioned above, an EGO sensor output signal is typically read by an electronic engine controller. Typical "front-end" circuitry into which each EGO sensor signal is routed in the electronic engine controller includes an operational amplifier. Such an operational amplifier's two power supply inputs are typically connected to a positive voltage power supply and to ground. The operational amplifier is typically configured in a "unity-gain" configuration such that the EGO sensor output signal is buffered by the operational amplifier but otherwise generally not changed. In such a unity-gain configuration, the EGO sensor output signal is routed into the non-inverting input of the operational amplifier. The output of the operational amplifier is then routed in some form to a microprocessor within the electronic engine controller. In general, the use of an operational amplifier in the front end of the electronic engine controller as described here is very economical in is thus desirable to use an operational amplifier for this application.
However, due to the use of an operational amplifier, a problem can occur when an EGO sensor experiences a CSD fault. This is due to a characteristic of a typical operational amplifier when connected to a positive voltage supply and to ground. As mentioned above, when an EGO sensor output signal experiences CSD, its voltage tends to shift negative. When a negative voltage of any more titan a small fraction of a volt is input into a typical operational amplifier connected to a positive voltage supply and to ground, the operational amplifier outputs a relatively large positive voltage. This positive voltage tends to approach the voltage of the positive voltage supply to the operational amplifier. This large positive voltage output from the operational amplifier is similar to the output which would occur if the EGO sensor output signal went into an overvoltage fault condition. Because the output from the operational amplifier is approximately the same when the EGO sensor experiences CSD as when the EGO sensor experiences an overvoltage fault, the electronic engine controller cannot typically tell the difference between the two conditions.
Because the electronic engine controller cannot generally tell the difference between a CSD fault and an overvoltage fault, a service technician working on the vehicle and interrogating the electronic engine controller's diagnostic memory has little or no guidance as to which condition occurred. As a result, the service technician may perform an incorrect or unnecessary repair procedure.
Therefore, means to detect CSD and to thereby distinguish it from other EGO sensor signal faults, while still employing an economical operational amplifier front end, will provide a great advantage over the prior art.